When the pipes are joined with flexible rings it is essential that the ring pressure against the two pipes should exceed the fluid pressure to which the rings are later subjected. Lip seal rings fulfil this requirement well and U or V cup packings (see FIG. 1) with a thin flexible lip establish a seal with only small initial jointing force while allowing large jointing tolerances with high sealing pressures. As the pipe fluid pressure always tends to spread the flanges of the seal, sealing pressure always exceeds the pipe fluid pressure.
In pipelines where each joint must be tested for fluid tightness against a stipulated pipeline pressure it is known to provide the female half of the spigot joint with a pair of seal ring grooves each receiving a sealing ring such as an "O" ring so that the space between the rings may be pressure tested to verify the integrity of each "O" ring. If a U or V cup packing is used (see FIGS. 2/3) and the male or spigot half of the joint is offered the first cup packing presents a trailing edge to the incoming pipe but the second presents a leading edge and jamming tends to result.
A characteristic of these lip seal joints is that when fluid pressure is applied to the interior of the ring the pressure assists the sealing action while when applied in the opposite direction the fluid pressure tends to lift the seal off its seating. This function is exemplified in a bicycle pump where the lip seal on the piston seals the air for pumping in the forward direction and acts as a non-return valve allowing air to pass in the reverse direction. See FIG. 2. From these characteristics of lip seals, it can be seen that for pipe sealing applications they are excellent seals able to span large gaps between the sealing faces but will only seal against fluid pressure in the direction towards the lips and will only allow a pipe spigot to be inserted towards the base of the ring.